Monolith joint waterstops are necessary in dams, power houses, navigation locks, and other structures, and these waterstops have been of several structures. In many instances, adjacent monoliths have different foundation support which may cause relative movement between these monoliths. Also, seasonal climatic changes can open and close the joints due to thermal expansion and contraction. Varying hydraulic loading conditions also effect movement. The joints must remain free to accommodate these movements. A common type of waterstop comprises embedded copper plates with a fold along the joint. More recently, embedded polyvinyl-chloride waterstops have been used which are inserted in vertical holes cut in straddling relation to the joint between the monoliths. Other types of waterstops have also been provided but in general all of the prior structures do not possess a combination of desired features, namely, a structure which makes them readily installable, including installation under water pressure conditions, which provides an effective seal, which accommodates relative movement between adjacent monoliths, which is long lasting and which is capable of being readily repaired or replaced. Remedial waterstops heretofore installed have not performed satisfactorily, not only for the same reasons surrounding the circumstances of installation, but for material failure as well.